The papillomaviruses are a group of viruses which have never been successfully propagated in tissue culture. Recently the molecular cloning of the viral genomes of selected papillomaviruses has permitted studies on their molecular biology. The bovine papillomaviruses are capable of transforming certain mouse fibroblast lines as well as certain rat fibroblast lines. The viral DNA in these transformed lines is maintained exclusively as extrachromosomal molecules. The extrachromosomal nature of the viral DNA in these lines together with the selected malignant phenotype suggested that the papillomaviruses may be a useful eukaryotic cloning vector. We have cloned the rat preproinsulin gene together with the transforming region of the bovine papillomavirus and assessed the ability of the papillomaviruses to be used as cloning vectors.